Here are the two scripts (you need some disk space to perform the dump / restore) . I assume you have copied the content of you "old" server on the new server. On Ubuntu/Debian servers, rrd files usually lives in /var/lib/munin.

First dump script :

#!/bin/bash
for f in `find /var/lib/munin -name '*.rrd' -print` ; do
    xml_file=`dirname $f`/`basename $f .rrd`.xml
   rrdtool dump "$f" > "${xml_file}"
   chown root:root "${xml_file}"
done

Import script :

#!/bin/bash
for f in `find /var/lib/munin -name '*.xml' -print` ; do
   rrd_file=`dirname $f`/`basename $f .xml`.rrd
   mv -f "${rrd_file}" "${rrd_file}.bak"
   chown root:root "${rrd_file}.bak"
   rrdtool restore "$f" "${rrd_file}"
   chown munin:munin "${rrd_file}"
done